Mobile or self-propelled work machines are known from practice, such as a mobile crane that has a drivable or mobile undercarriage on which an upper structure is rotatably mounted. In the upper structure, a diesel engine is arranged, for example, which can be used to drive hydraulic units of a crane boom and to drive a hydraulic motor that is arranged in the undercarriage and by means of which the drive wheels that are mounted on the undercarriage can be driven, making the work machine mobile. To transport the hydraulic energy into the undercarriage, a hydraulic rotary union is required. Hydraulic rotary unions are technically complex, expensive and accompanied by a considerable loss in efficiency.